Weighted open loop power control in a time division duplex communication system

ABSTRACT

The invention controls transmission power levels in a spread spectrum time division duplex communication station. A first communication station transmits a communication to a second communication station. The second station receives the communication and measures its received power level. Based on, in part, the received communication&#39;s power level and the communication&#39;s transmission power level, a path loss estimate is determined. A quality of the path loss estimate is also determined. The transmission power level for a communication from the second station to the first station is based on, in part, weighting the path loss estimate in response to the estimate&#39;s quality.

BACKGROUND

This invention generally relates to spread spectrum time division duplex(TDD) communication systems. More particularly, the present inventionrelates to a system and method for controlling transmission power withinTDD communication systems.

FIG. 1 depicts a wireless spread spectrum time division duplex (TDD)communication system. The system has a plurality of base stations 30₁-30 ₇. Each base station 30 ₁ communicates with user equipments (UEs)32 ₁-32 ₃ in its operating area. Communications transmitted from a basestation 30 ₁ to a UE 32 ₁ are referred to as downlink communications andcommunications transmitted from a UE 32 ₁ to a base station 30 ₁ arereferred to as uplink communications.

In addition to communicating over different frequency spectrums, spreadspectrum TDD systems carry multiple communications over the samespectrum. The multiple signals are distinguished by their respectivechip code sequences (codes). Also, to more efficiently use the spreadspectrum, TDD systems as illustrated in FIG. 2 use repeating frames 34divided into a number of time slots 36 ₁-36 _(n), such as fifteen timeslots. In such systems, a communication is sent in selected time slots36 ₁-36 _(n) using selected codes. Accordingly, one frame 34 is capableof carrying multiple communications distinguished by both time slot 36₁-36 _(n) and code. The use of a single code in a single time slot isreferred to as a resource unit. Based on the bandwidth required tosupport a communication, one or multiple resource units are assigned tothat communication.

Most TDD systems adaptively control transmission power levels. In a TDDsystem, many communications may share the same time slot and spectrum.When a UE 32 ₁ or base station 30 ₁ is receiving a specificcommunication, all the other communications using the same time slot andspectrum cause interference to the specific communication. Increasingthe transmission power level of one communication degrades the signalquality of all other communications within that time slot and spectrum.However, reducing the transmission power level too far results inundesirable signal to noise ratios (SNRs) and bit error rates (BERs) atthe receivers. To maintain both the signal quality of communications andlow transmission power levels, transmission power control is used.

One approach using transmission power control in a code divisionmultiple access (CDMA) communication system is described in U.S. Pat.No. 5,056,109 (Gilhousen et al.). A transmitter sends a communication toa particular receiver. Upon reception, the received signal power ismeasured. The received signal power is compared to a desired receivedsignal power. Based on the comparison, a control bit is sent to thetransmitter either increasing or decreasing transmission power by afixed amount. Since the receiver sends a control signal to thetransmitter to control the transmitter's power level, such power controltechniques are commonly referred to as closed loop.

Under certain conditions, the performance of closed loop systemsdegrades. For instance, if communications sent between a UE 32 ₁ and abase station 30 ₁ are in a highly dynamic environment, such as due tothe UE 32 ₁ moving, such systems may not be able to adapt fast enough tocompensate for the changes. The update rate of closed loop power controlin a typical TDD system is 100 cycles per second which is not sufficientfor fast fading channels. Accordingly, there is a need for alternateapproaches to maintain signal quality and low transmission power levels.

SUMMARY

The invention controls transmission power levels in a spread spectrumtime division duplex communication station. A first communicationstation transmits a communication to a second communication station. Thesecond station receives the communication and measures its receivedpower level. Based on, in part, the received communication's power leveland the communication's transmission power level, a path loss estimateis determined. The transmission power level for a communication from thesecond station to the first station is set based on, in part, weightingthe path loss estimate and a long term pathloss estimate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art TDD system.

FIG. 2 illustrates time slots in repeating frames of a TDD system.

FIG. 3 is a flow chart of weighted open loop power control.

FIG. 4 is a diagram of components of two communication stations usingweighted open loop power control.

FIG. 5 depicts a graph of the performance of a weighted open loop, openloop and closed loop power control system for a UE moving at 30kilometers per hour (km/h).

FIG. 6 depicts a graph of the three systems' performance for a UE movingat 60 km/h.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments will be described with reference to thedrawing figures where like numerals represent like elements throughout.Weighted open loop power control will be explained using the flow chartof FIG. 3 and the components of two simplified communication stations110, 112 as shown in FIG. 4. For the following discussion, thecommunication station having its transmitter's power controlled isreferred to as the transmitting station 112 and the communicationstation receiving power controlled communications is referred to as thereceiving station 110. Since weighted open loop power control may beused for uplink, downlink or both types of communications, thetransmitter having its power controlled may be located at a base station30 ₁, UE 32 ₁ or both. Accordingly, if both uplink and downlink powercontrol are used, the receiving and transmitting station's componentsare located at both the base station 30 ₁ and UE 32 ₁.

For use in estimating the path loss between the receiving andtransmitting stations 110, 112, the receiving station 110 sends acommunication to the transmitting station 112. The communication may besent on any one of various channels. Typically, in a TDD system, thechannels used for estimating path loss are referred to as referencechannels, although other channels may be used. If the receiving station110 is a base station 30 _(1,) the communication is preferably sent overa downlink common channel or a Common Control Physical Channel (CCPCH).

Data to be communicated to the transmitting station 112 over thereference channel is referred to as reference channel data. Thereference channel data is generated by a reference channel datagenerator 56. The reference data is assigned one or multiple resourceunits based on the communication's bandwidth requirements. A spreadingand training sequence insertion device 58 spreads the reference channeldata and makes the spread reference data time-multiplexed with atraining sequence in the appropriate time slots and codes of theassigned resource units. The resulting sequence is referred to as acommunication burst. The communication burst is subsequently amplifiedby an amplifier 60. The amplified communication burst may be summed by asum device 62 with any other communication burst created throughdevices, such as a data generator 50, spreading and training sequenceinsertion device 52 and amplifier 54. The summed communication burstsare modulated by a modulator 64. The modulated signal is passed throughan isolator 66 and radiated by an antenna 78 as shown or, alternately,through an antenna array, step 38. The radiated signal is passed througha wireless radio channel 80 to an antenna 82 of the transmitting station112. The type of modulation used for the transmitted communication canbe any of the those known to those skilled in the art, such as directphase shift keying (DPSK) or quadrature phase shift keying (QPSK).

The antenna 82 or, alternately, antenna array of the transmittingstation 112 receives various radio frequency signals. The receivedsignals are passed through an isolator 84 to a demodulator 86 to producea baseband signal. The baseband signal is processed, such as by achannel estimation device 88 and a data estimation device 90, in thetime slots and with the appropriate codes assigned to thecommunication's burst. The channel estimation device 88 commonly usesthe training sequence component in the baseband signal to providechannel information, such as channel impulse responses. The channelinformation is used by the data estimation device 90 and a powermeasurement device 92. The power level of the processed communicationcorresponding to the reference channel, R_(TS), is measured by the powermeasurement device 92 and sent to a pathloss estimation device 94, step40. The channel estimation device 88 is capable of separating thereference channel from all other channels. If an automatic gain controldevice or amplifier is used for processing the received signals, themeasured power level is adjusted to correct for the gain of thesedevices at either the power measurement device 92 or the pathlossestimation device 94.

To determine the path loss, L, the transmitting station 112 alsorequires the communication's transmitted power level, T_(RS). Thetransmitted power level, T_(RS), may be sent along with thecommunication's data or in a signaling channel. If the power level,T_(RS), is sent along with the communication's data, the data estimationdevice 90 interprets the power level and sends the interpreted powerlevel to the pathloss estimation device 94. If the receiving station 110is a base station 30 ₁, preferably the transmitted power level, T_(RS),is sent via the broadcast channel (BCH) from the base station 30 ₁. Bysubtracting the received communication's power level, R_(TS) in dB, fromthe sent communication's transmitted power level, T_(RS) in dB, thepathloss estimation device 94 estimates the path loss, L, between thetwo stations 110, 112, step 42. Additionally, a long term average of thepathloss, L₀, is updated, step 44. In certain situations, instead oftransmitting the transmitted power level, T_(RS), the receiving station110 may transmit a reference for the transmitted power level. In thatcase, the pathloss estimation device 94 provides reference levels forthe path loss, L, and the long term average of the path loss, L₀.

Since TDD systems transmit downlink and uplink communications in thesame frequency spectrum, the conditions these communications experienceare similar. This phenomenon is referred to as reciprocity. Due toreciprocity, the path loss experienced for the downlink will also beexperienced for the uplink and vice versa. By adding the estimated pathloss to a desired received power level, a transmission power level for acommunication from the transmitting station 112 to the receiving station110 is determined. This power control technique is referred to as openloop power control.

Open loop systems have drawbacks. If a time delay exists between theestimated path loss and the transmitted communication, the path lossexperienced by the transmitted communication may differ from thecalculated loss. In TDD where communications are sent in differing timeslots 36 ₁-36 _(n), the time slot delay between received and transmittedcommunications may degrade the performance of an open loop power controlsystem. To overcome these drawbacks, a quality measurement device 96 ina weighted open loop power controller 100 determines the quality of theestimated path loss, step 46. The quality measurement device 96 alsoweights the estimated path loss, L, and long term average of thepathloss, L₀, to set the transmit power level by transmit powercalculation device 98, step 48. As illustrated in FIG. 4, the weightedopen loop power controller 100 consists of the power measurement device92, pathloss estimation device 94, quality measurement device 96, andtransmit power calculation device 98.

The following is one of the preferred weighted open loop power controlalgorithms. The transmitting station's power level in decibels, P_(TS),is determined using Equation 1.

P _(TS) =P _(RS)+α(L−L ₀)+L ₀+CONSTANT VALUE  Equation 1

P_(RS) is the power level that the receiving station 110 desires toreceive the transmitting station's communication in dB. P_(RS) isdetermined by the desired SIR, SIR_(TARGET), at the receiving station110 and the interference level, I_(RS), at the receiving station 110.

To determine the interference level, I_(RS), at the receiving station,received communications from the transmitting station 112 aredemodulated by a demodulator 68. The resulting baseband signal isprocessed, such as by a channel estimation device 70 and a dataestimation device 72 in the time slots and with the appropriate codesassigned the transmitting station's communications. The channelinformation produced by the channel estimation device 70 is used by aninterference measurement device 74 to determine the interference level,I_(RS). The channel information may also be used to control the transmitpower level of the receiving station 110. The channel information isinput to a data estimation device 72 and a transmit power calculationdevice 76. The data estimation produced by the data estimation device 72is used with the channel information by the transmit power calculationdevice 76 to control the amplifier 54 which controls the receivingstation's transmit power level.

P_(RS) is determined using Equation 2.

P _(RS) =SIR _(TARGET) +I _(RS)  Equation 2

I_(RS) is either signaled or broadcasted from the receiving station 110to the transmitting station 112. For downlink power control,SIR_(TARGET) is known at the transmitting station 112. For uplink powercontrol, SIR_(TARGET) is signaled from the receiving station 110 to thetransmitting station 112. Using Equation 2, Equation 1 is rewritten aseither Equations 3 or 4.

P _(TS) =SIR _(TARGET) +I _(RS)+α(L−L ₀)+L ₀+CONSTANT VALUE  Equation 3

P _(TS) =αL+(1−α)L _(0+I) _(RS) +SIR _(TARGET)+CONSTANT VALUE  Equation4

L is the path loss estimate in decibels, T_(RS)-R_(TS), for the mostrecent time slot 36 ₁-36 _(n) that the path loss was estimated. The longterm average of the pathloss, L₀, is a running average of the path lossestimates L. The CONSTANT VALUE is a correction term. The CONSTANT VALUEcorrects for differences in the uplink and downlink channels, such as tocompensate for differences in uplink and downlink gain. Additionally,the CONSTANT VALUE may provide correction if the transmit powerreference level of the receiving station is transmitted, instead of theactual transmit power, T_(RS). If the receiving station is a basestation 30 ₁, the CONSTANT VALUE is preferably sent via Layer 3signaling.

The weighting value, α, determined by the quality measurement device 94,is a measure of the quality of the estimated path loss and is,preferably, based on the number of time slots 36 ₁-36 _(n) between thetime slot, n, of the last path loss estimate and the first time slot ofthe communication transmitted by the transmitting station 112. The valueof a is from zero to one. Generally, if the time difference between thetime slots is small, the recent path loss estimate will be fairlyaccurate and a is set at a value close to one. By contrast, if the timedifference is large, the path loss estimate may not be accurate and thelong term average path loss measurement is most likely a better estimatefor the path loss. Accordingly, α is set at a value closer to zero.

Equations 5 and 6 are two equations for determining a, although othersmay be used.

α=1−(D−1)/(D _(max)−1)  Equation 5

α=max{1−(D−1)/(D _(max-allowed)31 1), 0}  Equation 6

The value, D, is the number of time slots 36 ₁-36 _(n) between the timeslot of the last path loss estimate and the first time slot of thetransmitted communication which will be referred to as the time slotdelay. If the delay is one time slot, α is one. D_(max)is the maximalpossible delay. A typical value for a frame having fifteen time slots isseven. If the delay is D_(max), α is zero. D_(max-allowed) is themaximum allowed time slot delay for using open loop power control. Ifthe delay exceeds D_(max-allowed), the long term average pathlossmeasurement, L₀, is considered the better estimate for the pathloss andα=0. Using the transmit power level, P_(TS), determined by a transmitpower calculation device 98, the weighted open loop power controller 100sets the transmit power of the transmitted communication, step 48.

Data to be transmitted in a communication from the transmitting station112 is produced by a data generator 102. The communication data isspread and time-multiplexed with a training sequence by the spreadingand training sequence insertion device 104 in the appropriate time slotsand codes of the assigned resource units. The spread signal is amplifiedby the amplifier 106 and modulated by the modulator 108 to radiofrequency.

The weighted open loop power controller 100 controls the gain of theamplifier 106 to achieve the determined transmit power level, P_(TS),for the communication. The communication is passed through the isolator84 and radiated by the antenna 82.

FIGS. 5 and 6 depict graphs 82,84 illustrating the performance of aweighted open loop system using Equation 4. Equation 5 is used tocalculate α. These graphs 82, 84 depict the results of simulationscomparing the performance of a weighted open loop, an open loop and aclosed loop system controlling the transmission power level of thetransmitting station 112. The simulations address the performance ofthese systems in a fast fading channel under steady-state conditions. Inthis example, the receiving station is a base station 30, and thetransmitting station is a UE 32 ₁. For the simulation, the UE 32 ₁ was amobile station. The simulated base station 30 ₁ used two antennadiversity for reception with each antenna having a three finger RAKEreceiver. The simulation approximated a realistic channel and SIRestimation based on a midamble sequence of burst type 1 field in thepresence of additive white Gaussian noise (AWGN). The simulation used anInternational Telecommunication Union (ITU) Pedestrian B type channeland QPSK modulation. Interference levels were assumed to be accuratelyknown with no uncertainty. Channel coding schemes were not considered.The CONSTANT VALUE and L₀ were set at 0 db.

For each of the power control techniques, FIG. 5, graph 82 shows theenergy for a transmitted complex symbol in decibels (Es/No) required tomaintain a BER of 1% for various time slot delays, D, with the UE 32 ₁moving at 30 kilometers per hour (km/h). As shown, at lower time slotdelays, both weighted open loop and open loop outperform closed loop.For higher time slot delays, weighted open loop outperforms both openloop and closed loop. As shown in FIG. 6, graph 84, similar resultsoccur if the UE 32 ₁ is traveling at 60 km/h.

What is claimed is:
 1. A method for controlling transmission powerlevels in a spread spectrum time division duplex communication systemhaving frames with time slots for communication, the method comprising:transmitting a first communication having a transmission power level ina first time slot from a first communication station; receiving at asecond communication station the first communication and measuring apower level of the first communication as received; determining a pathloss estimate based on in part the measured received first communicationpower level and the first communication transmission power level;setting a transmission power level for a second communication in asecond time slot from the second station to the first station based onin part combining the path loss estimate weighted by a first factor witha long term path loss estimate weighted by a second factor, wherein thefirst and second factors are a function of a time separation of thefirst and second time slots; and determining a quality, α of the pathloss estimate based on in part a number of time slots, D, between thefirst and second time slot; and wherein the first factor is α and thesecond factor is 1−α.
 2. The method of claim 1 further comprising:determining the long term path loss estimate based at least in part uponan average of path loss estimates of communications sent from the firststation to the second station.
 3. The method of claim 1 wherein amaximum time slot delay is D_(max) and the determined quality, α, isdetermined by α=1−(D−1)/(D _(max)−1).
 4. The method of claim 1 wherein amaximum allowed time slot delay is D_(max-allowed) and the determinedquality, α, is determined by α=max{1−(D−1)/(D _(max-allowed)−1),0}. 5.The method of claim 1 wherein the set transmission power levelcompensates for differences in the uplink and downlink gains.
 6. Themethod of claim 1 wherein the first station is a base station and thesecond station is a user equipment.
 7. The method of claim 1 wherein thefirst station is a user equipment and the second station is a basestation.
 8. A spread spectrum time division duplex communication systemhaving a first and second communication station, the system using frameswith time slots for communication, the system comprising: the firststation comprising: means for transmitting a first communication havinga transmit power level in a first time slot; and the second stationcomprising: means for receiving the first communication and measuring apower level of the first communication as received; means fordetermining a path loss estimate based on in part the measured receivedfirst communication power level and the first communication transmissionpower level; means for setting a transmission power level for a secondcommunication in a second time slot based on in part combining the pathloss estimate weighted by a first factor with a long term path lossestimate weighted by a second factor, wherein the first and secondfactors are a function of a time separation of the first and second timeslots; means for transmitting the second communication in the secondtime slot at the set transmission power level; and means for determininga quality, α, of the path loss estimate based on in part a number oftime slots, D, between the first and second time slot; and wherein thefirst factor is α and the second factor is 1−α.
 9. The system of claim 8wherein the second station further comprises: means for determining thelong term path loss estimate based at least in part upon an average ofpath loss estimates of communications sent from the first station to thesecond station.
 10. The system of claim 8 wherein a maximum time slotdelay is D_(max) and the determined quality, α, is determined byα=1−(D−1)/(D _(max)−1).
 11. The system of claim 8 wherein a maximumallowed time slot delay is D_(max-allowed) and the determined quality,α, is determined by α=max{1−(D−1)/(D_(max-allowed)−1),0}.
 12. The systemof claim 8 herein the setting means sets the transmission power level tocompensate for differences in the uplink and downlink gains.
 13. Thesystem of claim 8 herein the first station is a base station and thesecond station is a user equipment.
 14. The system of claim 8 whereinthe first station is a user equipment and the second station is a basestation.
 15. A communication station having its transmission power levelcontrolled in a spread spectrum time division duplex communicationsystem, the system using frames with time slots for communication andhaving a second communication station transmitting a first communicationin a first time slot, the communication station comprising: at least oneantenna for receiving the first communication and transmitting anamplified second communication in a second time slot; a channelestimation device having an input configured to receive the receivedfirst communication for producing channel information; a data estimationdevice having inputs configured to receive the received firstcommunication and the channel information for producing interpreteddata; a power measurement device having an input configured to receivethe channel information for producing a measurement of a received powerlevel of the first communication; a pathloss estimation device having aninput configured to receive the measured received power level forproducing a pathloss estimate for the first communication; a qualitymeasurement device for producing a quality measurement based at least inpart upon a time separation of the first time slot and a second timeslot; a transmit power calculation device having inputs configured toreceive the pathloss estimation and the quality measurement forproducing a power control signal based at least in part upon combiningthe path loss estimate weighted by a first factor and a long termpathloss estimate weighted by a second factor, wherein the first andsecond factors are based on in part the quality measurement; and anamplifier having inputs configured to receive the power control signaland a second communication to be transmitted in the second time slot foramplifying the second communication in response to the power controlsignal to produce the amplified second communication.
 16. Thecommunication station of claim 15 further comprising: a data generatorfor producing communication data; a spreading and training sequenceinsertion device having an input configured to receive the communicationdata for producing the second communication in the second time slot; anda modulator having an input configured to receive the amplified secondcommunication for modulating the amplified second communication to radiofrequency prior to transmission.
 17. The communication station of claim15 further comprising: a demodulator having an input configured toreceive the received first communication for producing a basebandsignal; and wherein both the channel estimation device and the dataestimation device have an input configured to receive the basebandsignal.
 18. The communication station of claim 15 wherein the qualitymeasurement is in the range of zero to one and the first factor is thequality measurement and the second factor is one minus the qualitymeasurement.
 19. The communication station of claim 15 wherein the firststation is a base station and the second station is a user equipment.20. The communication station of claim 15 wherein the first station is auser equipment and the second station is a base station.